C++

Background

Mechanisms for exception handling were added to C++ in 1990. Exceptions themselves are user- or library- defined – the language itself does not provide any actual exceptions, just means of handling them.

Catching Exceptions

C++ exceptions follow the form:

try {
    // code that may raise an exception
} catch (/* exception parameter */) {
    // handler code
} catch (/* exception parameter */) {
    // handler code
}
// ...

catch is the name of all handlers in C++ – it is an overloaded name, so the exception parameter of each must be unique. You do not need to specify a variable name to store the value of the parameter, you can simply specify the type – e.g. catch(int), as opposed to catch(int i). A parameter variable is often useful, though, as it transfers information to the handling code. The parameter may also simply be an ellipses: catch(...), which is an exception catch-all (compare to Java).

After a handler completes its execution, control flows to the first statement after the try/catch block that handled the exception.

Throwing Exceptions

Exceptions are explicitly raised by a throw statement:

throw expression;

where expression may be any type (compare to Java).

A throw without an argument can only appear in a handler – this simply re-raises the exception, which should be handled elsewhere.

try {
    // code that may raise an exception
} catch (string s) {
    // handler for string exceptions
} catch (...) {
    // throw all other exceptions
    throw;
}

Concrete Example

int num;
char ch;
double dNum;

try {
    std::cout << "Enter int from 1 - 100: ";
    std::cin >> num;
    if (num < 1 || num > 100) {
        throw num;
    }

    std::cout << "Enter 'A' or 'B' : ";
    std::cin >> ch;
    if (ch != 'A' && ch != 'B') {
        throw ch;
    }

    std::cout << "Enter double 1.1 - 1.5: ";
    std::cin >> dNum;
    if (dNum < 1.1 || dNum > 1.5) {
        throw dNum;
    }
} catch (int numIn) {
    std::cout << numIn << "is not in the specified range\n";
} catch (char charIn) {
    std::cout << charIn << "is not A or B\n";
} catch (...) {
    std::cout << "Invalid input!\n";
}

Unhandled Exceptions and Propagation

An unhandled exception is propagated to the calling function, and this propagation continues up to the main function if it goes unhandled. If no matching handler is found through the entire propagation, the default handler (unexpected) is called. This default handler simply terminates the program. (Note: The default handler may be replaced by a user-defined handler, which must be a void function that takes no parameters).

Propagation Example

int doInput() {
    int num;
    std::cout << "Enter a number from 1-100: ";
    std::cin >> num;
    if (num < 1 || num > 100) {
        throw num; 
    }
    return num;
}

int main() {
    int num;
    try {
        doInput() 
    } catch (int numIn) {
        std::cout << numIn << " is not in the range\n"; 
        system("pause");
        exit(1);
    }

    std::cout "Continuing on...\n";
    system("pause");
}

Download the example program and run it to see out it works. To build, try g++ exceptions.cpp.

Exercise

Note: You can skip this exercise for now so that you have time to get through all of the class exercise today, but be sure to come back and complete this later.

Modify the program so that it has another exception named FailingInputException. If the user inputs a grade less than 60, the program should display a message that we don’t allow failing grades and then continue accepting grades. The totals should be revised so that only 4 totals are displayed.

Original code execution

Please input a grade or -1 to end: 100
Please input a grade or -1 to end: 50
Please input a grade or -1 to end: 67
Please input a grade or -1 to end: 92
Please input a grade or -1 to end: -1
End of input data reached
Limits Frequency
0 9 0
10 19 0
20 29 0
30 39 0
40 49 0
50 59 1
60 69 1
70 79 0
80 89 0
90 100 2
Press any key to continue . . .

Revised code execution

Please input a grade or -1 to end: 100
Please input a grade or -1 to end: 50
We don't allow failing grades!
Continuing with grades
Please input a grade or -1 to end: 67
Please input a grade or -1 to end: 92
Please input a grade or -1 to end: -1
End of input data reached
Limits Frequency
60 69 1
70 79 0
80 89 0
90 100 2
Press any key to continue . . .

Note: This is not a good use of exceptions. The point is really to contrast the mechanics of C++ and Java.

Hint

There are two try/catch blocks. Be sure to add your new exception to the correct one.

dgrisham.github.io

C++